Pulse synchronizing means for multiroute p. c. m. system



S. E. MILLER Jan. 14, 1964 PULSE SYNCHRONIZING MEANS FOR MULTIROUTEP.C.M. SYSTEM Filed Deo. l, 1960- THDOM, Ok hQO? kwvm Oh AMS:

A TTORNEV United States Patent O Tais invention relates to thederivation of timing pulses in pulse communication systems and moreparticularly to the derivation at points of intersection in multiroutepulse modulation systems of timing pulses suitable for use in reducingthe signals of the intersecting routes to a common time base.

A complex cross country communication system having intersecting signalroutes must be capable of transferring information from one route toanother at the intersecting stations. Por example, it may be desired totransmit information from Chicago to Philadelphia. if a facility existsfor transmitting information from Chicago to New York and another fromNew York to Philadelphia, ernploying these facilities and transferringthe information at New Yori; obviates the need for a direct route fromChicago to Philadelphia. By practicing this principle in an extendedcross country system the number of direct routes required between pointsis greatly diminished.

Pulse modulation communication systems are no exception to the generalrule insofar as information transfer at intersecting stations isconcerned. Pulse signals of a large number of information channels areusually multiplexed on a single communication route on a time basis. Atthe intersecting stations or branching points the pulses from thechannels carried on the various routes are interleaved with one another,or substituted one for the other, effecting a transfer of informationfrom route to route.

In an actual system the pulse signals on the intersecting routesencounter widely diiferent conditions in traveling to the branchingpoint and consequently may be expected to occur asynchronously at thebranching point. rhese signals, therefore, must first be brought intosynchronism or reduced to a common time base prior to being interleaved.Heretofore, synchronisrn has been achieved by retiming all the signalscarried on the various intersecting routes with respect to a single setof timing pulses usually derived from a portion of the signal on anarbitrarily chosen one of the intersecting routes by frequency selectionof the basis pulse repetition frequency thereof.

Such systems are inherently subject to the disadvantage that failure ofthe signal route upon which the arbitrarily chosen pulse informationsignal is carried incapacitates the entire retirning operation at thebranching point.

Moreover, although the timing pulses follow in phase the pulses carriedon the chosen route, this is not true of the pulse signals on the otherintersecting routes. Misalignment, i.c., difference between the time ofoccurrence of the timing and information pulses, may become excessive ifthe Sigi als from the chosen route deviate greatly in phase from thoseof the other intersecting routes. This gives rise to possible errors inretiming in that spurious pulses may be created or information pulsesdestroyed.

lt is, therefore, the object of the present invention to derive a set oftiming pulses in a multiroute pulse communication system whose existenceis indierent to failure of any of the individual routes and to realizeimproved alignment of such pulses with respect to the signal pulsescarried on the individual routes.

In accordance with the above object, the timing scheine ofthe presentinvention derives a single set of timing pulses by frequency selectionfrom the combination of Patented dan. lfl,

portions of pulse information signals from all the intersecting routes.The contribution of signals from all the routes in deriving a set oftiming pulses insures the continuation of the retiming operation despitethe cessation of signals on any individual routes. Furthermore, sincethe pulse signals from all the intersecting routes contribute to thephase characteristics of the set of derived timing pulses, these timingpulses are more closely aligned with the information pulse signals andhence the possibility of excessive misalignment between timing pulsesand any of the individual pulse information signals is markedly reduced.

More specifically po11ions of the pulse signals carried on theindividual routes are abstracted from the routes and applied in parallelto a band-pass filter. The bandpass filter selects the component of thebasic repetition frequency of each pulse information signal, and thesecomponents add to form a composite signal which may be shaped into a setof timing pulses, representative in phase of the information signalsfrom all the intersecting routes. Utilization circuits individual toeach route retime the pulse signal thereon under the control of theunitary set of timing pulses. At this point the pulse informationsignals of the various routes occur in synchronism and hence areamenable to interleaving.

As an additional feature to improve alignment between signal and timingpulses, variable delay devices individual to each signal route controlthe delay introduced into each route prior to the application of signalstherein to any combining or utilization circuit. An error signal isdeveloped for each route which is indicative of the phase discrepancybetween the timing pulses and the signal pulses on that route. Thissignal is employed to control the delay introduced into its associatedsignal route by the variable delay device.

The above and other features of the invention are considered in detailin the following specification taken in conjunction with the drawing,the single ligure of which delineates a branching point repeaterillustrating the features of the present invention.

The branching point repeater of the ligure is adapted to receive radiofrequency pulse information signals. More specifically the signals mightbe pulse code modulated information from a plurality of channels alltime multiplexed onto one radio frequency signal route for the purposeof radio transmission. The present invention may be practiced, however,with equal success in a baseband pulse code system. Signals l@ from aroute traveling west to east and signals .ll from a route travelingnorth to south are intercepted by antennas 12 constituting the irststages of identical radio frequency signal paths shown in the figure asheavy lines. The intercepted signals ld, considering the West to eastsignal path as exemplary of both signal paths, are applied to a variabledelay device la, to be described in more detail subsequently, andthereafter to an amplifier i6. A. di- 'rectional coupler itl abstracts aportion of the radio frequency signal from the output of amplifier 16.The remainder of the output of amplifier lo is applied to the input of aregenerator 2d.

An envelope detector 26 demodulates the radio frequency signalabstracted by directional coupler 18. The baseband pulses developed byenvelope detector 25 are applied to a high Q, band-pass iilter 34, tunedto the basic pulse frequency of the system, together with similar pulsesabstracted from the other radio frequency signal path. Bandpass filter3e) selects and transmits the components of the basic pulse frequency ofthe signals applied thereto. These basic pulse frequency components addat the output of bandpass filter 34 to form a composite timing wave. Thebasic pulse frequencyy as reairain ferred Vto herein is the number ofpulses which would occur per second if a pulse were present in each timeposition of the pulse coded signal.

' Since the signals from both routes are utilized in de- ?riving thetiming pulses, failure of signals upon either route does not interruptthe timing operation at the branching point repeater. The compositetiming Wav iis applied to a shaping circuit 36 which forms pulses to'satisfy the system requirements.

l ln many radio frequency pulse code systems, for instance, it is founddesirable to derive timing pulses having a raised cosine shape. Toperform this function, shaping circuit 35 could comprise a peak limiter44, a low-Q, band-pass filter le and a base limiter i3 connected intandem. The peak limiter removes any amplitude variations from thecomposite timing wave and filter #to eliminates the harmonics generatedby the limiting operation. A base limiter develops raised cosine pulsesfrom the output of band-pass filter do. Examples of possible circuitswhich might be employed as limiters 44 and i3 are illustrated in section4-3 of Pulse and Digital Cir- 'fcuits by Millman and Taub, McGraw-TillBook Comlpany, Inc., 1956.

The resultant set of timing pulses emanating from shaping circuit 3e isapplied to regenerator 20 through a fixed delay 32. The purpose of fixeddelay 3?. is to align the centers of the timing pulses and the signalpulses applied to regenerator 2li on a steady-state basis.

Timing pulses are applied to regenerator 26 to control the retiming ofthe radio frequency signal pulses traversing the radio frequency signalpath. Patent 2,868,965 which issued January 13, 1959 to O. E. DeLangediscloses a regenerator which may be advantageously employed asregenerator 2Q. The regenerated radio frequency pulse signals areamplified in an amplifier 22 after which they are introduced toappropriate multiplex circuitry indicated by a block dll.

Y As described in connection with the west to east signal tl, the northto south signal 11 also traverses the radio frequency signal path towhich it is assigned and is applied to multiplex circuitry 4d. Because acommon set of timing pulses was employed to retime the informationsignals of both routes, the pulse signals on the routes occur insynchronism and hence, signals from channels yon the west to east routemay be interleaved with or substituted for the signals from the channelson the north to south route, and vice versa, to bring about a transferof information at the branching point.

`The regrouped multiplex signals are applied to their respective signalpaths from which they, as represented oy a west to east signal 42 and anorth to south signal e3, are radiated by output antennas 24.

The timing wave emanating from band-pass filter 34 is a sine waveoscillating at the basic pulse frequency of the system and having aphase relative to the pulse code signals transmitted on the intersectingroutes which is the average of the phase difference between the basicpulse frequency components of the information signals carried on thecontributing routes. This fact is evidenced Eby consideration of thewell-known trigonometric relaetionship which dictates that the sum oftwo sine waves having the same equency but differing in phase is a :sinewave having the same frequency as the addend sine waves but a phaserelative to the addend sine waves `which is half the phase difference ofthe addend sine waves. See College Algebra and Trigonometry by FredericH. Miller, John Wiley and Sons, Inc., 1945 at Chapter 6, Article 39,Equation (7) for a symbolic representation of this trigonometricidentity. Hence, there is a limitation upon the .system that the basicpulse frequency of the pulse signals on the contributing routes must beidentical.

The principles of this invention may be extended to include any numberof signal routes without losing the advantages enumerated for the exe.piary embodiment.

lt should be noted, however, that if a timing Wave which has a properlyweighted average or arithmetic mean phase is desired for some particularreason, the number of contributing routes should be an integral power of2, that is, 2, 4, 8, i6, etc.

By deriving the timing wave which is at worst indicative of the phaserelation of the signals on all the routes, the alignment between thetiming pulses and the information pulses from any of the individualroutes is improved.

The alignment is further improved by -a control circuit for applyingvariable delay to each radi-o frequency signal path by means of variabledelay devices 14. Thus in each route, a portion of the demodulated pulsecode signal emanating from envelope detector 26 is compared in phasewith a portion of the set of timing pulses emanating from shapingcircuit 36 in a phase detector 28 and a comparison signal developed.Control to variable delay device illiis ar'orded by the phase comparisonsignal developed by phase detector 28.

The correction rendered by the variable delay control circuit may beconsidered to be of two types. First, it lcorrects for misalignment ofinformation and timing pulses resulting from fast variations in pulsepositions of the information pulses, called time jitter. Thismisalignment occurs due to the slow response of band-pass ritter 34 tojitter, and is a phenomenon found in prior art applications employing aband-pass filter to derive a timinv Wave from one information pulsesignal. Second, the control circuit compensates for misalignment betweentiming pulses and the individual signal pulses which occur because ofthe fact that the timing pulses re not derived from the pulse signals ona single route but represent an average of the pulse signals from aplurality of signal routes. This is, of course, characteristic of theapplication of a band-pass filter found in the present invention.

By way of example, the function of variable delay de 'ice 14 andamplifier 16 could be carried out with a traveling wave tube. Thecontrol in this case would be applied to the helix element of thetraveling wave tube to effect a change in the helix-to-cathode potentialand hence vary the delay experienced by the radio yfrequency signalwhile traversing the traveling wave tube. A more detailed description ofthe operation of this circuit may be had by reference to the copendingapplication of W. M. Goodall, Serial No. 70,219, filed November i8,196i), now Patent No. 3,t)'f85,2tl(l, assigned to the assignee of thepresent application.

Also by way of example phase detector 28 might conveniently be asillustrated in FIGS. 12-14 of Electronic Instruments, RadiationLaboratory Series, volume 21, McGraw-Hill Book Company, Inc., 1948. lfthis circuit were employed the envelope detector output could be appliedas the error signal and the timing pulses as the reference voltage.

What is claimed is:

l. in a multiroute communication system, a plurality of signal routes,each carrying pulse information signals, means for obtaining a sample ofthe information pulses carried by each of said routes, a frequencyselective device having a band-pass characteristic related to a chosenrepetitive characteristic of said signals, means for applying saidsamples to said frequency selective device, means Afor shaping theoutput of said frequency selective device into timing pulses,utilization circuits individual to each of said routes and tandemlyconnected therewith, and means for applying said timing pulses to saidutilization circuits to govern timing operations thereat.

2. A multiroute communication system as defined in claim l wherein theinformation pulses carried on said signal routes each have a basicrepetition frequency of fo and said frequency selective device is tunedto the frequency fo.

3. ln a communication system, a plurality of signal routes, eachcarrying pulse information signals, means for obtaining a sample of theinformation pulses carried by each of said routes, a band-pass filtertuned to the basic repetition frequency of said signals, means `forapplying each of said samples to said band-pass filter, means forshaping the output of said band-pass lter to form a set of timingpulses, a utilization circuit individual to each of said routes andtandemly connected therewith, means for applying said timing pulses tosaid utilization circuits to `govern timing operations thereat, avariable delay device individual to each of said routes and tandemlyconnected therewith ahead of said utilization circuit, means forcomparing the phase of said timing pulses With said sample from each ofsaid routes to develop individual control signals representative of themisalignment of the information signals carried on each of said routes,and means for applying each of said control signals to its respectivedelay device to control the delay introduced into that signal route inorder to bring the information pulses applied to said utilizationcircuits into closer alignment with said timing pulses.

4. -In a self-timed regenerative repeater, apparatus for reducing pulseinformation signals carried by a plurality of signal routes to a commontime base, said apparatus comprising means for obtaining a sample of theinformation pulses transmitted on each of said routes, a frequencyselective device having a band-pass characteristic related to a chosenrepetitive characteristic of said signals, means for applying each fsaid samples simultaneously to said frequency selective device, meansfor shaping said frequency selective device output to form timingpulses, a pulse regenerator individual to each of said routes andtandemly connected therewith, and means for applying said timing pulsesto each of said regenerators to control the retirning of the pulseinformation signals applied thereto.

5. A regenerative repeater as defined in claim 4 in which the basicpulse repetition frequency of the information pulses carried by each ofsaid routes is fo and said frequency selective device is tuned to fu.

6. A self-timed regenerative repeater for regenerating radio frequencypulse information signals carried lby at least two signal routes tooccur in phase synchronization comprising means for sampling the radiofrequency signals being transmitted on each of said routes, means fordemodulating each of said samples, a band-pass filter tuned to the basicrepetition frequency of said signals, means for applying saiddemodulated samples to said band-pass filter, means for shaping saidband-pass filter output into a set of timing pulses, a radio frequencypulse regenerator individual to each of said routes and tandemlyconnected therewith, means for applying said timing pulses to each ofsaid regenerators to control the retiming of radio frequency pulsesignals applied thereto.

7. A self-timed regenerative repeater for regenerating at least twopulse information signals to occur in phase synchronization comprising asignal path for each of said signals including in order an inputcircuit, a variable delay device, a pulse regenerator for producingoutput pulses in response to input pulses, the time of occurrence andamplitude of said output pulses depending upon the time of occurrence ofcontrol pulses and the amplitude of said input pulses at said time ofoccurrence of said control pulses, respectively, and an output circuit,means for obtaining samples of the information pulses occurring at theoutput of said delay device in each of said paths, a band-pass filtertuned to the basic repetition frequency of said signals, means forapplying said samples to said vfilter to derive a set of timing pulses,the phase of which represents the phases of said sample signals, meansfor applying said set of timing pulses to said regenerators, in each ofsaid signal paths to control the retiming therein, means for comparingsaid set of pulses and said samples in each path to derive individualcontrol quantities for the respective variable delay devices, and meansfor applying each of said control quantities to its respective delaydevice to control the delay introduced into said signal paths.

8. A self-timed regenerative repeater for relegating to a common timebase pulse information signals carried by a plurality of signal routescomprising means for obtaining a sample of the information pulses beingtransmitted by each of said routes, a band-pass filter tuned to thebasic repetition frequency of said signals, means for applying saidsamples to said band-pass filter, means for shaping said band-passfilter output to form a set of timing pulses, a pulse regeneratorindividual to each of said channels and tandemly connected therewith,means for applying said timing pulses to each of said regenerators tocontrol the retiming of the pulse information signal applied thereto, avariable delay device individual to each of said routes and tandemlyconnected therewith ahead of said regenerator, means for developing acontrol signal for each of said routes representative of the phasedifference between said sample from its respective route and said timingpulses, and means for applying each of said control signals to itsrespective delay device to control the delay introduced into that signalroute in order to bring the information signals to be regenerated intoalignment with said timing pulses.

9. In a multiroute communication system, a plurality 0f signal routes,said plurality being equal to an integral power of 2, each routecarrying pulse information signals having the same basic repetitionfrequency, means for obtaining a set of timing pulses occurring at saidbasic repetition frequency and equal in phase to the average of each ofsaid pulse information signals carried on said signal routes,utilization circuits individual to each of said routes and tandemlyconnected therewith, and means for applying said timing pulses to saidutilization circuits to govern timing operations thereat.

References Cited in the lile of this patent UNITED STATES PATENTS2,949,503 Andrews Aug. 16, 1960

1. IN A MULTIROUTE COMMUNICATION SYSTEM, A PLURALITY OF SIGNAL ROUTES,EACH CARRYING PULSE INFORMATION SIGNALS, MEANS FOR OBTAINING A SAMPLE OFTHE INFORMATION PULSES CARRIED BY EACH OF SAID ROUTES, A FREQUENCYSELECTIVE DEVICE HAVING A BAND-PASS CHARACTERISTIC RELATED TO A CHOSENREPETITIVE CHARACTERISTIC OF SAID SIGNALS, MEANS FOR APPLYING SAIDSAMPLES TO SAID FREQUENCY SELECTIVE DEVICE,